Rapid advances in laser technology have enabled various techniques for the generation and detection of electromagnetic radiation in the terahertz region (spanning from ˜100 GHz to ˜10 THz, or wavelengths between ˜30 μm and ˜3 mm). As a result, numerous uses of terahertz radiation have been explored, including trace gas detection, medical diagnosis, security screening, and defect analysis in complex materials such as space shuttle tiles. Many of these studies have relied on terahertz time-domain spectroscopy, a technique for generating sub-picosecond pulses with spectral content spanning much of the THz band.
However, progress is limited by the overwhelming reliance on free-space transport of the terahertz beam, using bulk optical components. In many real-world situations, the sample or region to be studied may not be readily accessible to a line-of-sight beam. Hence, common devices that operate at other wavelengths, such as optical fiber-based sensors or medical endoscopes rely on the guided wave delivery of light to the remote sensing location. In addition, while THz waves can be transmitted simply by free space propagation, free space propagation requires bulk optical components, which are difficult to align.
Thus, in order to expand the usefulness of THz radiation, it is desirable to provide optimized guided wave devices that operate at THz frequencies. The development of practical THz waveguides would dramatically expand the application of THz-TDS in areas such as gas sensing and nanometer thin-film measurements.
Heretofore, the development of THz waveguides has been hindered by the material properties and the application requirements in this spectral range. On the one hand, the characteristics of materials at THz frequencies make it extremely difficult to build a fiber to guide THz beams over a long distance. The most transparent materials for this range are crystalline (e.g., high resistivity silicon), and thus are costly, fragile, and challenging to form into specific geometries for waveguide configurations. Other materials, such as low-loss polymers or glasses, are more malleable but exhibit prohibitively high absorption losses for propagation distances of more than a few centimeters. For this reason, THz waveguides generally must rely on propagation in air, rather than via dielectric confinement as in an optical fiber.
On the other hand, many THz applications rely on the use of broadband pulses for time-domain analysis and spectroscopic applications. To avoid pulse reshaping during propagation, low dispersion is required. But for many conventional metal waveguides (e.g., metal tubes), pulse reshaping in propagation is difficult to avoid, due to the extreme dispersion near the waveguide cutoff frequencies. Furthermore, finite conductivity of metals can lead to considerable losses in the wave propagation.
Great efforts have been devoted to finding useful THz waveguides within the last few years, and various guides with quasi-optical coupling have been demonstrated. Most of these THz waveguides have been based on conventional guiding structures, such as metal tubes, plastic ribbons, or dielectric fibers. There have also been reports on the application of the latest technology of photonic crystal fibers to THz radiation. In all of these cases, the utility for transport of THz pulses is limited by group velocity dispersion of the guided waves.
The most promising studies have reported dispersionless propagation in parallel metal plate waveguides. One type of dispersionless waveguide design has been discussed recently by Grischkowsky and co-workers. This design is a ribbon waveguide, which is dispersionless and low-loss. In the Grischkowsky design, the loss is attributable to two factors: (1) lateral spreading due to the fact that the mode is unconfined in one of the two transverse dimensions, and (2) the finite conductivity of the metal used to confine the mode, which in this case results in a reported attenuation of ˜80 dB/m.
Coaxial waveguides, have not heretofore been considered, due to the difficulties in coupling the radiation into the guide. This is because linearly or circularly polarized light cannot be effectively coupled into a coaxial waveguide. In particular, coaxial waveguides have not been used previously at frequencies above a few GHz, because of the difficulties in coupling the radiation into the waveguide efficiently. Hence, it remains desired to provide a waveguide that is effective at THz frequencies.